Saddle-shaped aza-nanographene with multiple odd-membered rings

Abstract

A saddle-shaped aza-nanographene containing a central 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) has been prepared via a rationally designed four-step synthetic pathway encompassing intramolecular direct arylation, the Scholl reaction, and finally photo-induced radical cyclization. The target non-alternant, nitrogen-embedded polycyclic aromatic hydrocarbon (PAH) incorporates two abutting pentagons between four adjacent heptagons forming unique 7−7−5−5−7−7 topology. Such a combination of odd-membered-ring defects entails a negative Gaussian curvature within its surface with a significant distortion from planarity (saddle height ≈ 4.3 Å). Its absorption and fluorescence maxima are located in the orange-red region, with weak emission originating from the intramolecular charge-transfer character of a low-energy absorption band. Cyclic voltammetry measurements revealed that this stable under ambient conditions aza-nanographene underwent three fully reversible oxidation steps (two one-electron followed by one two-electron) with an exceptionally low first oxidation potential of E_ox1 = −0.38 V (vs. Fc/Fc⁺).